Large amounts of sulfur dioxide are emitted annually by industrial plants worldwide and strict legislation to control these emissions is being enacted in most countries. Wet scrubbing systems, such as spray towers, are the most common means employed for removing sulfur dioxide from flue gas. The wet scrubbers typically use an aqueous solution or slurry of an alkaline reagent, such as sodium bicarbonate, sodium carbonate, lime or limestone, to remove the SO.sub.2 from the flue gas. These solutions and slurries of alkaline reagents are often referred to as "SO.sub.2 absorbent" or simply as "absorbent." While the conventional wet scrubbers have proved to be a fairly effective means of removing SO.sub.2, they tend to have high maintenance costs due to excessive scale build-up. Additionally, SO.sub.2 removal efficiency of the conventional wet scrubbers is only about 90%, which is not always adequate to enable the emitted flue gas to meet regulatory standards in many areas around the world.
Tank scrubbers offer an alternative to the conventional wet scrubbers. The tank scrubbers generally provide a vessel containing an aqueous slurry or solution of an alkaline reagent, usually either lime or limestone, to serve as the absorbent for SO.sub.2. The flue gas is injected directly into the absorbent slurry. Several devices have been proposed to contact the SO.sub.2 -containing gas with the absorbent slurry. For example, in U.S. Pat. No. 4,099,925, the flue gas is contacted with the absorption slurry in a liquid-raising pipe. In U.S. Pat. No. 4,156,712 the flue gas is sparged into an upper portion of the liquid absorbent without the aid of any mechanical agitation above the level of flue gas injection. In U.S. Pat. No. 4,229,417, flue gas dispersion is achieved in a manner similar to that of the '712 patent except that the pipes feeding the flue gas into the absorbent slurry have notches to provide for better gas dispersion. U.S. Pat. No. 4,911,901 discloses a process wherein the flue gas is contacted with the spray of the absorbent slurry prior to gas-slurry contact in the reactor. The SO.sub.2 removal efficiencies attained by these prior art tank scrubbers are in the range of 90-99% which is a considerable improvement over conventional wet scrubber efficiencies. The efficiency of these tank scrubbers is still not always adequate to meet regulatory requirements, particularly in cases where the SO.sub.2 concentration in the flue gas is relatively high, and more particularly where the gas contains greater than 1% SO.sub.2.
Tank scrubbers generally include a device to bubble the flue gas through the absorbent. As a result, the tank scrubbers typically are limited with respect to the volume of flue gas that can be treated.
Although the current use of tank scrubbers has been limited due to their difficulty in handling very large volumes of flue gas, there are many potential applications for the tank scrubbers in industrial plants which produce relatively small volumes, i.e., less than 25,000 CFM, of flue gas with a high SO.sub.2 content. For example, newly developed metallurgical processes, such as oxygen-enriched gold ore roasting, produce such gases. Small Claus plants also produce such gases. Due to the high efficiency of the tank scrubbers when used to treat low volumes of flue gases, the number of potential applications should grow as environmental regulations become increasingly stringent. Continued improvements in tank scrubber design would provide even more opportunities for the use of the tank scrubbers.
There is therefore a continual need for improving the efficiency of tank scrubbers to remove SO.sub.2 from flue gas and other gas streams. There is further a particular need for improved tank scrubbers that are able to remove large amounts of SO.sub.2 from low-volume streams of flue gas containing high concentrations of SO.sub.2.
The present invention is accordingly directed to a method and apparatus for providing an improved tank scrubber design for removing SO.sub.2 from flue gas. The process and apparatus of the invention is able to remove SO.sub.2 effectively from low-volume streams of flue gas containing high concentrations of SO.sub.2.